Electrically Conductive Adhesives

Electrically conductive adhesive products are primarily used for electronics applications where components must be held in place, and electrical current can be passed between them.

Depending on the gap between components, most general adhesives (such as anaerobics, cyanoacrylates, epoxies, and acrylic-based adhesives) act as an electrical insulator. In addition, some offer improved thermal conductivity to help with thermal management of electronic components and heat sinks, directing heat away from sensitive components. Because in many cases (particularly when using an anaerobic or cyanoacrylate adhesive) there is no glue line control and effectively parts are touching (with adhesives filling in microscopic crevices), some electrical charge can still be transferred as there is enough metal to metal contact still occurring.

Certain temperature-sensitive electronic components cannot be soldered because the intense heat of liquid solder and the soldering iron can cause damage to the component. This type of application calls for an electrically conductive adhesive that can be used instead of solder. PCBs with components attached to both sides can also benefit from using an electrically conductive adhesive as the assembly process is easier without the risk of components dropping off the underside when parts are soldered on the top. When using electrically conductive adhesive for an entire electrical assembly, it negates the requirement to undergo a solder re-flow process.

Applications for electrically conductive adhesives aren’t just limited to bonding components onto PCBs or die attach; they can be very useful for other electronic applications where substrates are temperature sensitive – such as for touch-panels, LCD displays, coating and bonding RFID chips, and mounting LEDs. Solar cells also use adhesives instead of solder as there is less warpage and damage to the sensitive wafers that make up solar cells.

Selecting an Electrically Conductive Adhesive

There are some vital points to consider when selecting an electrically conductive adhesive:

• To begin with, the level of electrical conductivity (or volume resistivity).
• Also, the viscosity and rheology of the adhesive. Whether it needs to flow well or stand as a proud drop (with high “wet” strength).
• And then there is the filler particle size – what is acceptable or necessary?
• Curing mechanism and cure speed – how do you plan to cure the adhesive e.g. two component mix and then room temperature cure, or heat cure – if the application involves temperature-sensitive components, is a heat cure suitable? How quickly does the adhesive need to cure?
• Production line considerations. Firstly, how fast is the throughput? Secondly, is the process fully automated or manual? How will you dispense the adhesive?
• Nature of the materials being bonded and level of adhesion required. For example, joint design, the strength required, any differential thermal expansion and contraction, thermal conductivity, glass transition temperature, and flexibility requirements.
• Environmental service conditions such as temperature, exposure to chemicals, humidity, etc.
• Tests that the adhesive must pass e.g. drop tests, accelerated aging tests.
• Color, smell, health and safety considerations, shipping, storage, and shelf life.
• And not forgetting one of the most important considerations – cost!

Types of Electrically Conductive Adhesive

Electrically conductive adhesive can be based on several different chemistries:

• Electrically conductive silicone adhesives can be graphite filled and are often used for EMI/RFI shielding or for antistatic systems. These materials are generally a very high viscosity, thick consistency making them suited to larger applications such as gasketing or bonding / sealing large areas. The electrical conductivity is fairly limited (so they are not a good replacement for solder). Volume resistivity is typically about 0.09 Ohms∙cm.
• Two-component epoxy adhesives comprise resin and hardener and are available in a range of viscosities. However, if heavily filled with conductive metal, viscosity can become quite high. If filled with silver, volume resistivity can be as low as 0.0001 Ohms∙cm.
• One component epoxy adhesives are normally heat cured so take care to choose a cure schedule that won’t compromise sensitive electronic components. Snap-cure frozen epoxies are also popular for the electronics industry; these products require freezer storage and cure once they reach room temperature. These can be expensive to transport and store. A silver filled one-part epoxy can achieve conductivity as high as a similarly filled two-part epoxy.
• Silver-filled polyurethane adhesives are starting to appear on the market. They are two-part adhesives, so they either require mixing or they are supplied pre-mixed and frozen like the snap-cure epoxies. They offer high peel strength and flexibility. As they are silver filled, high levels of conductivity can be achieved (around 0.0001 Ohms∙cm to 0.0004 Ohms∙cm).

Developing an Electrically Conductive Adhesive

As with many things in life, there are certain trade-offs. In the case of electrically conductive adhesive they are:

Conductive fillers can be considered as follows:

Electrical Properties of Adhesives – Terminology

What are conductivity, resistivity, and dielectric strength, how are they tested, and what do the measurements mean? Looking at technical datasheets comparing products can be very confusing. The industry uses so many different units of measurement, and it is very difficult to compare competitor products when nobody is using the same test methods or UOM. For this reason, it is always a good idea to test the adhesives to check suitability rather than condemn an adhesive based on comparing technical datasheets.

Dielectric Strength

This pertains to electrically insulating adhesive i.e. the electricity must not be conducted. Many potting and encapsulation applications require an epoxy glue with high dielectric strength.

This is the maximum voltage the adhesive can withstand before it is destroyed. In fact, it is also known as “breakdown voltage” for reasons which explain themselves.

The standard industry test is ASTM D-149. Because results are affected by the adhesive thickness and the temperature at which the testing is done. It is important to compare like-for-like!

In the US, dielectric strength is often specified as volts per mil (a thousandth of an inch). Whereas elsewhere, it is mainly V/cm (or mm or m)

To convert:

• 1 V/m = 2.54 x 10^-5 V/mil
• 1 V/mil = 3.94 x 10⁴ V/m
• 1 V/m = 0.001 V/mm
• 1 V/mm = 1000 V/m
• 1 V/mm = 1 kV/m
• 1 kV/mm = 1000 kV/m

As a comparison, typical dielectric strengths of different adhesive types are:

Dielectric Constant

This is the ability of the adhesive to store a charge (electric flux). This is affected by the temperature, and also the glass transition temperature (Tg) of the adhesive as insulating properties change above and below the Tg. So, the higher the Tg, the better the retention of dielectric properties at elevated temperatures. Typical values for insulating epoxy glues are around 4 to 6 at around 1 mHz.

Volume Resistivity

This measures the electrical conductivity or electrical resistance of materials, taking into account sample dimensions (hence the “volume” part). Test standards associated with this are the old MIL STD-883, ASTM D2739, and ASTM D257-99, a test method for measuring DC resistance or conductance of insulating materials. Units of measurement associated with Volume resistivity are typically Ohm∙cm. Accordingly, the lower the figure, the more electrically conductive the adhesive is.

What do isotropic and anisotropic mean in relation to electrically conductive adhesives?

Isotropic conductive adhesives are electrically conductive in all directions and are therefore ideal for die attach, chip bonding, attaching SMDs, etc. Anisotropic conductive adhesives only conduct electricity in one direction, so these are often used for very sensitive electronic components such as LEDs, LCDs, and RFIDs.

For further help and advice, product recommendations, and information about Permabond’s adhesives for electronic components, please feel free to contact Permabond and we will arrange for our chemists to assist you further.

Click to download Permabond’s Adhesive for Electronics brochure.